Method of making a bistable micromagnetic light modulator

ABSTRACT

A method of fabricating a modulator for modulating an incident beam of light includes a substrate having a cavity formed therein and a plurality of spaced-apart deformable elements formed in the cavity. The deformable elements each has a base layer, a poled magnetic layer formed in the base layer and a first light reflection layer deposited on the magnetic layer for reflecting an incident beam of light. Between adjacent deformable elements on the base of the cavity is arranged a second light reflection layer. A conductive element formed in the substrate electro-magnetically energizes the deformable elements to deflect in the cavity. Incident light passing through each one of the first light reflection layers is caused to destructively interfere with light reflected from the second light reflection layers thereby causing modulation of the incident light.

FIELD OF THE INVENTION

This invention relates to a method of fabricating a modulator formodulating a beam of light. More particularly, this invention describessuch a method that utilizes a substantially flat reflective surfacehaving selectively deformable elements for providing a diffractiongrating.

BACKGROUND OF THE INVENTION

Advances in micromachining technology have given rise to a variety ofMicro-electromechanical systems (MEMS) including light modulators forlow cost display applications. Such modulators provide high-resolution,high operating speeds (KHz frame rates), multiple gray scale levels,color adaptability, high contrast ratio, and compatibility with VLSItechnology. One such modulator has been disclosed in U.S. Pat. No.5,311,360, titled "Method and Apparatus for Modulating a Light Beam,"issued May 10, 1994, by Bloom et al. This modulator is a micromachinedreflective phase grating. It consists of a plurality of equally spaceddeformable elements in the form of beams suspended at both ends above asubstrate thereby forming a grating. The deformable elements have ametallic layer that serves both as an electrode and as reflectivesurface for incident light. The substrate is also reflective andcontains a separate electrode. The deformable elements are designed tohave a thickness equal to λ/4 where λ is the wavelength of the incidentlight source. They are supported a distance of λ/4 above, and parallelto, the substrate. Thus, when the deformable elements are unactivated,i.e., undeflected, the distance between their top surface and thesubstrate equals λ/2. Thus, when light impinges perpendicularly to thesurface of this surface the grating reflects light as a flat mirror.However, when a sufficient voltage (switching voltage) is appliedbetween the deformable elements and the substrate, the resultingelectrostatic force pulls a portion of the deformable elements down adistance λ/4 toward the substrate, thereby reducing the distance betweenthe top of this portion of the elements and the substrate to λ/4. Thus,light reflected from this portion of the deformable elements is out ofphase with that from the substrate and a diffraction pattern is formed.Optical systems can intercept the diffracted light with output occurringonly when the deformable elements are activated (i.e., pulled down). Fordisplay applications, a number of deformable elements are grouped forsimultaneous activation thereby defining a pixel, and arrays of suchpixels are used to form an image.

U.S. Pat. No. 5,677,783 titled "Method of Making a Deformable GratingApparatus for Modulating a Light Beam and Including Means for ObviatingStiction Between Grating Elements and Underlying Substrate," issued Oct.14, 1997, by Bloom et al. discloses a modulator which obviates stictionbetween grating elements and underlying substrate. One problem with theprior art modulator is that it is activated via an electrostatic forcewhich is nonlinear. Specifically, as the voltage applied to themodulator increases from zero, the activated deformable elements deflectincrementally until they reach approximately 1/3 of their full scaledeflection, and then they jump the remaining distance until they impactthe substrate. Therefore, when the prior art modulator modulates light,the activated deformable elements contact the substrate and this givesrise to significant stiction problems.

Therefore, a need exists for a method of fabricating a modulator inwhich the deformable elements provided can be held stationary at anypoint over the entire range of their motion so that light modulation canoccur without the deformable elements contacting the substrate therebyeliminating the stiction problem.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide amodulator for modulating an incident beam of light by providing adeformable structure which can effectively move the desired λ/4distance.

This and other objects of the invention are accomplished with a methodof fabricating a bistable micromagnetic modulator for modulating anincident beam of light, comprising the steps of:

providing a substrate comprising a cavity having a base and side wallssurrounding said base, wherein said base comprises at least oneferromagnetic element arranged substantially lengthwise therein;

providing a plurality of equally spaced apart, deformable elementssuspended above said cavity in a first position, each one of saiddeformable elements having opposing end edges integrally formed in saidside walls of said substrate just above said cavity, wherein each one ofsaid plurality of deformable elements comprises a base layer having arecess, a poled magnetic layer disposed in said recess, and a firstlight reflection layer deposited on said magnetic layer for reflectingsaid incident beam of light;

providing at least one conductive element arranged substantiallylengthwise in one of said side walls surrounding said cavity in anelectro-magnetic relationship to said magnetic layer of said pluralityof equally spaced apart deformable elements;

providing a plurality of second light reflection layers arranged on thebase of said cavity, a single one of said plurality of second lightreflection layers being arranged between nearest adjacent spaced apartdeformable elements; and

providing means for applying a current through said conductive element,said current producing a magnetic field in the deformable element whichcauses said deformable elements to deflect to a second position towardssaid ferromagnetic element in said cavity such that each one of saidpoled magnetic layers in said plurality of deformable elements inducespoles in said ferromagnetic element thereby producing an attractivemagnetic force between said magnetic layer and said ferromagneticelement, said attractive magnetic force holding said plurality ofdeformable elements in said second position such that light reflectingfrom said plurality of first light reflection layers destructivelyinterferes with light reflected from said plurality of second lightreflection layers thereby causing modulation of said incident light.

An advantage of the method of fabricating the light modulator of theinvention is that the deformable elements provided can be deflected overthe entire range of their possible motion thereby accommodating a rangeof incident wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent when taken in conjunction with thefollowing description and drawings wherein identical reference numeralshave been used, where possible, to designate identical features that arecommon to the figures, and wherein:

FIG. 1 is a perspective, partially cut-away view of a modulator of theinvention;

FIG. 2 is a sectional view of the modulator of FIG. 1 taken along lineA--A of FIG. 1 wherein the deformable elements are in the up position;

FIG. 3 is a sectional view of the modulator of FIG. 1 taken along lineA--A of FIG. 1 wherein the deformable elements are in the down position;

FIG. 4 is a perspective view of a layer of hard magnetic material whichhas been polarized along its length;

FIG. 5 is a sectional view of the modulator of FIG. 1 taken along lineB--B of FIG. 1 wherein the deformable elements are in the up position;

FIG. 6 is a sectional view of the modulator of FIG. 1 taken along lineB--B of FIG. 1 wherein the deformable elements are being displaced tothe down position via an applied current;

FIG. 7 is a sectional view of the modulator of FIG. 1 taken along lineB--B of FIG. 1 wherein the deformable elements are held in the downposition with no applied current;

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1, 2, and 3, perspective, partially cut-away, andsectional views of a light modulator 10 of the invention are shown. Thelight modulator 10 comprises a plurality of equally spaced deformableelements 12 in the form of beams. The substrate 14 has a cavity 32 thathas a base 30 and side walls 44, and the deformable elements 12 aresupported at both ends above the cavity 32 with both ends integrallyformed in the side walls 44. The deformable elements 12 comprise a baselayer 16, preferably silicon nitride, having a recess 34, a layer ofhard magnetic material 18 is deposited in the recess 34, and a firstlight reflection layer 20, preferable aluminum, is deposited on the topof the layer 18 of hard magnetic material, as shown. The layer 18 ofhard magnetic material is preferably made from cobalt-platinum (Co-Pt)which is deposited for in plane polarization at room temperature usingdc or rf magnetron sputtering as described in the publication entitled"Structure and Micromagnetic Predictions for Hysteretic Phenomena in aNovel Co-Pt Permanent Magnetic Thin Film," by R. H. Victora, et al. inJournal of Magnetism and Magnetic Materials, Vol. 97, 1991, pp. 343-352.The layer of hard magnetic material 18 is polarized along its length(see FIG. 4).

There are conductive elements 22 and 24 arranged in the side walls 44 onsubstrate 14 in proximity to the plurality of deformable elements 12 asshown. The conductive elements 22 and 24 are connected to power sources26 and 28, respectively. There is a plurality of second light reflectionlayers 36, one each being arranged on the base 30 of the cavity 32between the spaced apart deformable elements 12 as shown. In addition,ferromagnetic element 50 is arranged in the substrate 14 beneath thebase 30 of cavity 32 as shown. The ferromagnetic element 50 runs thelength of the substrate, and is centered below the south and north polesof the layer 18 of hard magnetic material on the plurality of deformableelements 12. The ferromagnetic element 50 is preferably made of softmagnetic materials including permalloy, supermalloy, sendust, iron,nickel, nickel-iron, or alloys thereof.

In FIG. 2 the modulator 10 is shown in a sectional view taken along lineA--A of FIG. 1. The modulator 10 is shown with the power sources 26 and28 off so that there is no current flowing through conductive elements22 and 24. When no current flows through conductive elements 22 and 24,the deformable elements 12 are flat (i.e., in an up position) due to theinherent residual tensile stress therein. The modulator 10 is designedso that when a light wave 40 of wavelength λ impinges perpendicularly tothe surface of the modulator 10, the light reflected from the firstlight reflection layer 20 on the deformable elements 12 is in phase withthe light reflected from the plurality of second light reflection layers36 on the base 30 of the cavity 32 between the deformable elements 12and consequently, the modulator 10 reflects light as a flat mirror asindicated by arrow 38.

In FIG. 3 the modulator 10 is shown in a sectional view taken along lineA--A of FIG. 1. The power sources 26 and 28 are turned on therebycausing currents to flow in conductive elements 22 and 24 as will bedescribed. The applied currents gives rise to magnetic fields thatimpart a Lorentz force to the magnetic poles in the layer of hardmagnetic material 18 in the deformable elements 12 which is sufficientto bend the deformable elements 12 downward until the midportion of thedeformable elements 12 deflects a distance λ/4 downward (see FIG. 6).Thus, when a lightwave 40 of wavelength λ impinges perpendicularly tothe surface of the modulator 10, the light reflected from the firstlight reflection layer 20 on the deformable elements 12 is out of phasewith the light reflected from the plurality of second light reflectionlayers 36 on the base 30 of the cavity 32 between the deformableelements 12, and the modulator 10 diffracts the incident light indirections indicated by arrows 42. Optical systems can be designed tointercept the diffracted light with output occurring only when thedeformable elements 12 are activated. For display applications, a groupof deformable elements 12 can be simultaneously activated to form apixel, and arrays of such pixels can be fabricated for displaying animage.

Referring to FIG. 4, a perspective view is shown of a polarized layer 18of hard magnetic material in isolation. As shown in FIG. 1, magneticlayer 18 comprising this hard magnetic material is disposed in recess 34of each one of the deformable elements 12.

Referring to FIG. 5, a sectional view is shown of the modulator 10 takenalong line B--B of FIG. 1, wherein the deformable elements 12 are in anunactivated up position (i.e., power sources 26 and 28 are off).

Referring to FIG. 6, a sectional view is shown of the modulator 10 takenalong line B--B of FIG. 1, wherein the deformable elements 12 are in anactivated down position, i.e., power sources 26 and 28 are turned on.Specifically, to activate the deformable elements 12, the power sources26 and 28 cause currents to flow through conductive elements 22 and 24,in a direction out of the paper as indicated by current arrows tips 100as is well known. The current flowing through the conductive element 22gives rise to a magnetic field indicated by field line 110 which impartsa downward Lorentz force to the south pole of the layer of hard magneticmaterial 18. The current flowing through the conductive element 24 givesrise to a magnetic field indicated by field line 120 which imparts adownward Lorentz force to the north pole of the layer of hard magneticmaterial 18. The currents in conductive elements 22 and 24 are ofsufficient magnitude to deflect the midportion of the deformableelements 12 downward a distance λ/4 as shown. It is instructive to notethat the modulator will modulate light if the deformable elements 12 aredeflected any odd multiple λ/4 i.e., 3λ/4, 5λ/4, 7λ/4, etc. It shouldalso be noted that the ferromagnetic element 50 aids in pulling down theplurality of deformable elements 12 due to the mutual magneticattraction of the south and north poles of the layer 18 of hard magneticmaterial in the plurality of deformable elements 12. It is important tonote that the activated deformable elements 12 obtain λ/4 the desireddeflection over a limited portion of their midsection due to the factthat deformable elements 12 are rigidly supported at both ends. When alightwave 40 of wavelength λ impinges perpendicularly to the surface ofthe modulator 10 when the deformable elements 12 are activated in thisfashion, the light reflected from the first light reflection layer 20 onthe midportion of the deformable elements 12 that is deflected downwarda distance λ/4 is in out of phase with the light reflected from theplurality of second light reflection layers 36 on the base 30 in thecavity 32 between the deformable elements 12, and the modulator 10diffracts the incident light as described above.

Referring to FIG. 7, a sectional view is shown of the modulator 10 takenalong line B--B of FIG. 1, wherein the deformable elements 12 are in anunactivated down position, i.e., power sources 26 and 28 are turned off.The deformable elements 12 are held in the down position due to theforce of attraction between the south and north poles of the layer 18 ofhard magnetic material and the ferromagnetic element 50. Specifically,the south pole of the layer 18 of hard magnetic material induces a northsurface pole 60 in the ferromagnetic element 50 which, in turn, impartsa downward force to the south pole of the layer 18 of hard magneticmaterial. Similarly, the north pole of the layer 18 of hard magneticmaterial induces a south surface pole 62 in the ferromagnetic element 50which, in turn, imparts a downward force to the south pole of the layer18 of hard magnetic material. These forces hold the plurality ofdeformable elements 12 in the down position even when no current flowsthrough the conductive elements 22 and 24. Thus the modulator 10 willcontinue to modulate light as described above when the deformableelements 12 are held down in this fashion even though it consumes noenergy.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

    ______________________________________                                        PARTS LIST                                                                    ______________________________________                                        10            light modulator                                                   12 deformable element                                                         14 substrate                                                                  16 baselayer                                                                  18 layer of hard magnetic material                                            20 first light reflection layer                                               22 conductive element                                                         24 conductive element                                                         26 power source                                                               28 power source                                                               30 base of cavity                                                             32 cavity                                                                     34 recess                                                                     36 second light reflection layer                                              38 light direction arrow                                                      40 light wave                                                                 42 light direction arrow                                                      44 side walls                                                                 50 ferromagnetic element                                                      60 north surface pole                                                         62 south surface pole                                                         100   tip of current                                                          110  magnetic field line                                                      120  magnetic field line                                                    ______________________________________                                    

What is claimed is:
 1. A method of fabricating a bistable micromagneticmodulator for modulating an incident beam of light, comprising the stepsof:providing a substrate comprising a cavity having a base and sidewalls surrounding said base, wherein said base comprises at least oneferromagnetic element arranged substantially lengthwise therein;providing a plurality of equally spaced apart, deformable elementssuspended above said cavity in a first position, each one of saiddeformable elements having opposing end edges integrally formed in saidside walls of said substrate just above said cavity, wherein each one ofsaid plurality of deformable elements comprises a base layer having arecess, a poled magnetic layer disposed in said recess, and a firstlight reflection layer deposited on said magnetic layer for reflectingsaid incident beam of light; providing at least one conductive elementarranged substantially lengthwise in one of said side walls surroundingsaid cavity in an electro-magnetic relationship to said magnetic layerof said plurality of equally spaced apart deformable elements; providinga plurality of second light reflection layers arranged on the base ofsaid cavity, a single one of said plurality of second light reflectionlayers being arranged between nearest adjacent spaced apart deformableelements; and providing means for applying a current through saidconductive element, said current producing a magnetic field in thedeformable element which causes said deformable elements to deflect to asecond position towards said ferromagnetic element in said cavity suchthat each one of said poled magnetic layers in said plurality ofdeformable elements induces poles in said ferromagnetic element therebyproducing an attractive magnetic force between said magnetic layer andsaid ferromagnetic element, said attractive magnetic force holding saidplurality of deformable elements in said second position such that lightreflecting from said plurality of first light reflection layersdestructively interferes with light reflected from said plurality ofsecond light reflection layers thereby causing modulation of saidincident light.
 2. The method recited in claim 1 wherein said step ofproviding a plurality of deformable elements further comprises the stepof providing said first reflection layer comprising materials selectedfrom the group consisting of: (a) aluminum, (b) copper, (c) gold, (d)silver, and, (e) alloys thereof.
 3. The method recited in claim 1wherein the step of providing at least one conductive element furthercomprises the step of providing said at least one conductive elementcomprising materials selected from the group consisting of (a) aluminum,(b) copper, (c) gold, (d) silver, and, (e) alloys thereof.
 4. The methodrecited in claim 1 wherein the step of providing a plurality ofdeformable elements further comprises the step of providing said poledmagnetic layer comprising cobalt-platinum.
 5. The method recited inclaim 1 wherein said step of providing a plurality of deformableelements further comprises the step of providing said base layercomprising silicon dioxide.
 6. The method recited in claim 1 whereinsaid step of providing a plurality of deformable elements furthercomprises the step of providing said base layer comprising siliconnitride.